This invention relates in general to fluid handling apparatus, and in particular to a dry, scroll-type, two-stage, positive displacement, vacuum pump useful in general roughing pump applications.
The general operating principles of scroll pumps are described in 1905 U.S. Pat. No. 801,182 to Creux. A movable spiral blade (sometimes termed a "wrap" or "wall") orbits with respect to a fixed spiral blade within a housing. The configuration of the blades and their relative motion traps one or more volumes or "pockets" of a fluid between the blades and moves the fluid through the pump. Creux describes using the energy of steam to drive the blades to produce a rotary power output. Most applications, however, apply a rotary power to pump a fluid through the device. Oil lubricated scroll pumps are widely used as refrigerant compressors. Other applications include expanders (operating in reverse from a compressor), and vacuum pumps. To date, scroll-type pumps have not been widely adopted for use as vacuum pumps.
Scroll pumps must satisfy a number of often competing design objectives. Blades must be configured to interact with each other so that their relative motion defines the pockets that transport, and often compress, the fluid held in the pockets. The blades must therefore move relative to each other, yet also seal. In vacuum pumping, the vacuum level achievable by the pump is often limited by the tendency of high pressure gas at the outlet to flow backwards toward the lower pressure inlet region. The effectiveness and durability of the blade seals, both tip seals along their spiral edges and clearance seals between fixed and movable blades, are important determinants of performance and reliability.
Friction in the drive, blade motion, and seals, as well as the compression of the working fluid, produce wear and heat. It is necessary to cool the apparatus. A wide variety of techniques are known. They include air cooling, flows of refrigerants, and flows or sprays of a lubricant which acts as a heat sink and transfer medium as well as a lubricant. Oil lubrication is the most common technique. Lubrication can also aid in sealing the movable component acting on the working fluid. However, when oil or other lubricants are used in vacuum pumps, as the pressure falls to low levels, the vapor pressure of the lubricant itself contributes lubricant to the gas which, to some degree, offsets the action of the pump. Vaporized lubricant can also flow back into the system being evacuated to contaminate the system with molecules of the lubricant.
Further, in vacuum pumping it is desirable to have a high volumetric displacement rate of gas from the vacuum region, e.g., to pump out quickly a mass spectrometer or a compartment of a machine where semiconductor devices are fabricated. In general, scroll designs for vacuum pumping produce little or no compression. But scroll pumps solely optimized for high displacement rates are often not well suited for operating across a large pressure differential, e.g., between a few milliTorr at the inlet and atmosphere, 760 Torr, at the outlet, and vice versa. To support a large pressure differential, it is known to use a blade pair with multiple revolutions which produce multiple blade surface-to-blade surface clearance seals that block a back flow of the fluid from the high pressure at the outlet. However, the through put, or displacement capacity, of such a pump is limited.
A seemingly straightforward solution to increasing displacement is to increase the maximum inter-blade spacing so each pocket has a larger volume. For a constant scroll wall thickness this spacing is set by the crank radius. Therefore displacement can, in theory, be increased merely by increasing the crank radius. However, a larger radius has various disadvantages such as an increase in seal velocity and attendant wear, an increase in the radial forces acting on the crank, and an increase in steady state power consumption which relates to seal velocity and friction. A larger crank radius also increases the diameter of the plate and therefore the overall dimensions of the pump. Also, for a given plate diameter, a large crank radius results in fewer revolutions, fewer clearance seals in series and, therefore, more back leakage. The seemingly simple solution of increasing the crank radius is therefore contraindicated by size, wear, and frictional heating considerations.
To increase pump capacity, it is also known to operate multiple scrolls in parallel as done by Iwata Air Compressor Corporation in its model ISP-500 dry scroll vacuum pump. This is a single stage roughing pump using two parallel, back-to-back scroll sets that each have blades with an angular extent of more than four revolutions. While this pump has a displacement rate of 20 cubic feet per minute (CFM) at its inlet, its pumping speed drops off markedly below 100 milliTorr, presumably due to back leakage through the pump from its outlet to its inlet. This is a quite significant problem in some applications, e.g., in some helium leak detectors, where the roughing pump inlet pressure must reach less than 20 milliTorr before the leak test can begin. Another problem is that this pump can achieve a base pressure of only 5 milliTorr, whereas, by way of comparison, a commercial two stage rotary, oil-lubricated roughing pump can produce base pressures of 0.5 milliTorr. Yet another problem is that this model Iwata pump uses about 20 feet of tip seal material. Wear of this amount of tip seal produces significant debris which can contaminate the system being evacuated. This amount of sealing material also adversely affects power requirements.
U.S. Pat. No. 3,802,809 to Vulliez discloses a two stage, scroll-type vacuum pump. The device is cooled, but not lubricated, by recirculating, pumped oil. This vacuum pump has an internal bellows and internal oil-carrying passages to isolate the scroll surfaces open to the working fluid from the oil cooling circuit. A drive at one off-center eccentric bearing propels a movable plate or plates. A two stage embodiment is shown, but it uses two movable plates. While Vulliez uses two stages with a nested first stage, the volumetric displacement rates of the stages are required to be equal (column 9, line 54). This arrangement limits the effective volumetric displacement rate attainable by the pump as a combined two stage unit. An in-built electric fan is disclosed as a possible cooling device, but it is auxiliary to the oil cooling circuit.
One recent scroll pump design combines scroll pumps in series to achieve improved operating results. For example, U.S. Pat. No. 5,304,047 to Shibamoto discloses a two stage, scroll-type, oil-lubricated refrigerant compressor. Shibamoto radially separates the inlet of the second stage from the outlet of the first stage. While Shibamoto discloses a two-stage pump, it is not suited for operation as a vacuum pump because it requires a dynamic, oil-lubricated seal at the outer edge of the orbiting second stage scroll to control back leakage of the gas. Also, oil coolant and lubricant is injected onto the moving parts in low and intermediate pressure zones, collected, and recirculated.
For extremely low pressure applications such as evacuating a spectrometer tube used in helium leak detectors, known scroll pumps are not operated alone. It is conventional to use a turbo pump in series with the scroll pump. This combination of pumps, scroll and turbo pumps, can produce the desired pressure levels in a commercially acceptable period of time, but this solution is expensive since a turbo pump typically costs several thousand U.S. dollars.
Still another design problem is how to synchronize the movement of the movable blades to produce the orbital motion while at the same time 1) resisting the substantial axial forces produced by the large pressure differentials within the pump, 2) avoiding contamination of the vessel being evacuated by lubricants or wear debris, and 3) avoiding corrosion of bearings within the pump by condensed water, water vapor or corrosive gases such as those used in semiconductor processing.
It is known to use idler cranks mounted between a housing and an orbiting plate. While known cranks are sturdy and comparatively inexpensive, they are not readily protected from corrosive process gases and they are not well-suited to bear a substantial axial load.
It is also known to use a purge gas, e.g., a flow of a dried inert gas such as nitrogen, to carry away and/or dilute corrosive gases and fluids that could attack the bearings. Water will corrode 52100 bearing steel commonly used for bearing races and balls. Ceramic balls and races avoid the problems but they are very costly and more susceptible to destruction when subjected to mechanical shocks or when they wear. A stainless steel such as type 440C is more tolerant of water, but the "aggressive" gases commonly used in semiconductor processing will attack it, as well as 52100 steel.
However, the implementation of purge gas flows in vacuum scroll pumps has been difficult. Heretofore purge gas flows been used mainly in connection with pumps other than scroll pumps, such as screw pumps, roots pumps, and claw pumps. For example, the commercial Iwata ISP series scroll pumps use idler cranks. They operate in a region that is at the pump intake pressure. If one were to flush these idlers with a purge gas, the pump would have to evacuate the purge gas as well as the gas in the system being evacuated. To adequately protect the idler cranks, the purge gas would thus significantly raise the pump inlet pressure.
It is also known to use elastomeric seals to block contaminants and corrosive gases from bearings and to hold lubricants against escape to the regions being pumped. Such seals are particularly common as shaft seals on main crankshaft bearings. The problem is that hot water or water vapor and corrosive gases leak under the seals over time due to pressure differentials that arise within the pump in its ordinary operation. These differentials are due to thermal transients and to changes in the pump inlet pressure, e.g., when a chamber to be evacuated is at atmospheric pressure and it is valved to an operating pump. Thermal transients are produced whenever the pump warms or cools. The gas pressure in trapped spaces around the bearings will rise or fall. Over time, the pressure differential across the seal equilibriates, causing a mass transport past the seal which can include water, water vapor or other corrosive gases.
It is therefore a principal object of this invention to provide a positive displacement, scroll-type, fluid handling device that has a high volumetric displacement rate at the inlet and which, when used as a vacuum pump, operates steady state between a milliTorr vacuum and atmosphere with a good control over fluid back leakage and with a purge gas flow over its bearings.
Another principal object of this invention is to provide these advantages while at the same time controlling bearing corrosion and lubricant leakage.
Another object is to provide a fluid handling device with the foregoing advantages that can operate with conventional idler cranks to provide synchronization.
A further object is to provide a fluid handling device with the foregoing advantages operated as a vacuum pump which can produce ultimate base pressures suitable for use in extremely low pressure applications such as directly evacuating the spectrometer tube in a helium leak detector.
Still another object is to provide a fluid handling apparatus with the foregoing advantages which uses only air cooling.
Another object is to provide a fluid handling device with the foregoing advantages that has a comparatively low cost of manufacture and good durability.